Electric generators are known to comprise a stator and a rotor.
The stator has slots housing conducting or stator bars (also called Roebel bars). The bars typically have a straight part (the central part) that is housed within the slots, and bent parts at the two opposite ends.
The bent parts have copper lugs brazed at their ends; the copper lugs are then brazed together to electrically connect the conducting bars together and form the armature or stator winding of the generator.
In order to protect and electrically insulate the reciprocally connected copper lugs, box-shaped caps are provided that cover the lugs; a putty is then provided to hold the caps in place and to guarantee a sufficient heat flow from the bar ends to the outside.
In this respect, U.S. Pat. No. 4,309,636, which is incorporated by reference, discloses a fiberglass reinforced epoxy or polyester resin cap, arranged to receive the lugs of conducting bar ends and connect them together via a putty. The putty is an epoxy resin with a quartz meal as a filler.
Neither the epoxy resin, nor the quartz meal exhibits a good thermal conductivity; therefore the thermal conductivity of the putty is fairly low (typically 0.25 W/mK).
DE 40 24 395 discloses insulating caps covering reciprocally connected copper lugs.
EP 0 713 281 discloses an insulation that is applied around the lugs and is impregnated during bar impregnation.
The general trend in generator building is to increase the capability of the generator, which is partly done by increasing the current density.
In order to keep the bar end temperature at tolerable values, the heat transport through cups and putty has to be increased.